The conventional paper shredder is generally composed of a microswitch or LED (light-emitting diode) electronic eye located at the paper feeding port of the paper shredder. The paper shredder is started as soon as the microswitch or electronic eye has detected the feeding of the paper to be shredded. Such a conventional sensing device as described above is capable of sensing the feeding of paper as soon as the front edge of the paper has entered the paper feeding port, thereby triggering the shredding action of the paper shredder. On the other hand, the conventional sensing device ceases to function as soon as the rear edge of the paper has passed the sensing device, thereby resulting in an immediate termination of the shredding action of the paper shredder. As a result, the rear end of the paper remains intact. In order to overcome such an operational deficiency as described above, the conventional paper shredder is provided with a delay circuit capable of cooperating with the microswitch or LED electronic eye in such a way that the operation of the cutter of the paper shredder is prolonged for 2-3 seconds so as to ensure that the paper is shredded in its entirety. However, the addition of the delay circuit results in an increase in the cost of the paper shredder.